Generally, an image sensor is a semiconductor device that converts an optic image to an electric signal. More specifically, a charge coupled device (CCD) is a device having a plurality of metal-oxide semiconductor (MOS) capacitors each formed within a proximate range from one another, and wherein a carrier electric charge is stored in and transmitted to each capacitor.
A charged coupled device (CCD) includes a plurality of photodiodes (PD), a plurality of vertical charge coupled devices (VCCDs), a horizontal charge coupled device (HCCD), and a sense amplifier. Herein, the photodiodes converting light signals to electric signals are aligned in a matrix form. The vertical charge coupled devices are formed between each of the photodiodes aligned in a matrix form and formed in a vertical direction to transmit electric charges generated from each photodiode in a vertical direction. The horizontal charge coupled device transmits the charges transmitted from the vertical charge coupled device in a horizontal direction, and the sense amplifier senses the charge transmitted in the horizontal direction and outputs the electric charges.
However, the above-described CCD is disadvantageous in that it has a complicated driving method, consumes a large amount of energy, and requires multiple photo processes, which complicates the fabrication process. In the CCD, a control circuit, a signal processing circuit, and an A/D converter circuit cannot be easily integrated to the CCD and, as a result, the device cannot be formed in a compact size.
Recently, to overcome such disadvantages of the CCD, a CMOS image sensor is considered to be the next generation image sensor. The CMOS image sensor adopts a CMOS technology, which uses the control circuit and the signal processing circuit as peripheral devices. The CMOS technology forms MOS transistors corresponding to the number of unit pixels on a semiconductor substrate. The CMOS image sensor is a device using a switching method that can sequentially detect the output of each unit pixel by using the MOS transistors. More specifically, by forming a photodiode and MOS transistors in each of the unit pixels, the CMOS image sensor can sequentially detect the electric signals of each unit pixel by using the switching method, thereby representing an image.
Also, because the CMOS image sensor uses the CMOS fabrication technology, the CMOS image sensor consumes less amount of energy, and uses a relatively simple fabrication process due to a smaller number of photo processes. Furthermore, in the CMOS image sensor, a control circuit, a signal processing circuit, an A/D converter circuit, etc., can be integrated to the CMOS image sensor chip, thereby allowing the CMOS image sensor to be formed in a compact size. Therefore, the CMOS image sensor is widely used in various applications, such as digital still cameras, digital video cameras, and the like.
Meanwhile, the CMOS image sensor can be divided into a 3-transistor (3T) type, a 4-transistor (4T) type, and a 5-transistor (5T) type CMOS image sensor depending upon the number of transistors used. The 3T type includes one photodiode and three transistors. The 4T type includes one photodiode and four transistors. And, the 5T type includes one photodiode and five transistors. An equivalent circuit and a layout of a unit pixel of the 3T type CMOS image sensor will now be described in detail.
FIG. 1 illustrates an equivalent circuit diagram of a known CMOS image sensor, and FIG. 2 illustrates a layout diagram of a known CMOS image sensor. As shown in FIG. 1, a unit pixel of the general 3T type CMOS image sensor includes one photodiode (PD) and three nMOS transistors (T1, T2, and T3). A cathode of the photodiode (PD) is connected to a drain of a first NMOS transistor (T1) and to a gate of a second NMOS transistor (T2). A source of each of the first and second transistors (T1 and T2) is connected to a power line, which provides a reference voltage (VR). A gate of the first NMOS transistor (T1) is connected to a reset line, which supplies a reset signal (RST). Also, a source of a third nMOS transistor (T3) is connected to a drain of the second NMOS transistor (T2). A drain of the third NMOS transistor (T3) is connected to a reader circuit (not shown) through a signal line. A gate of the third NMOS transistor (T3) is connected to a column select line, which provides a select signal (SLCT). Therefore, the first NMOS transistor (T1) will be referred to as a reset transistor (Rx), the second nMOS transistor (T2) will be referred to as a driver transistor (Dx), and the third NMOS transistor (T3) will be referred to as a select transistor (Sx).
Referring to FIG. 2, in the unit pixel of the general 3T type CMOS transistor, a photodiode 20 is formed on an active area and, most particularly, on a portion of the active area having a larger width. Gate electrodes 120, 130, and 140 of three transistors overlapping one another are formed on the remaining portions of the active area. More specifically, the gate electrode 120 forms the reset transistor (Rx), the gate transistor 130 forms the driver transistor (Dx), and the gate electrode 140 forms the select transistor (Sx). Herein, impurity ions are injected in the active area 10 of each transistor, except for the lower portions of the gate electrodes 120, 130, and 140, to form a source/drain area of each transistor. Therefore, a power voltage Vdd is applied to the source/drain area between the reset transistor (Rx) and the driver transistor (Dx), and a source/drain area on one side of the select transistor (Sx) is connected to the reader circuit (not shown).
As described above, although not shown in the drawings, each of the gate electrodes 120, 130, and 140 is connected to each signal line, and each of the signal lines is provided with a pad on one end to be connected to an external driving circuit. The signal lines provided with the pads and the following fabrication process will now be described in detail.
FIGS. 3A to 3E illustrate cross-sectional views of each signal line of a known CMOS image sensor and process steps of fabricating the known CMOS image sensor after forming the signal lines. Referring to FIG. 3A, an insulating layer 101 (e.g., an oxide layer), such as a gate insulating layer or an interlayer dielectric, is formed on a semiconductor substrate 100, and a metal pad 102 of each signal line is formed on the insulating layer 101. As shown in FIG. 2, the metal pad 102 may be formed of the same material as the gate electrodes 120, 130, and 140 and formed on the same layer. Alternatively, the metal pad 102 may also be formed of a different material through a separate contact and is usually formed of aluminum (Al). Also, a protective layer 103 is formed on an entire surface of the insulating layer 101 including the metal pad 102.
As shown in FIG. 3B, a photosensitive layer 104 is formed on the protective layer 103 and the photosensitive layer 104 is exposed and developed by using a photolithography process, thereby exposing an upper portion of the metal pad 102. Then, the protective layer 103 is selectively etched by using the photosensitive layer 104 as a mask to form an opening 105 on the metal pad 102. Finally, the photosensitive layer 104 is removed.
Referring to FIG. 3C, a first planarization layer 106 is deposited on the entire surface of the protective layer 103. Also, by treating the first planarization layer 106 with a photo-etching process by using a mask, only the portion of the metal pad region is removed. Then, a blue color filter layer 107, a green color filter layer 108, and a red color filter layer 109 are serially formed on the first planarization layer 106 corresponding to each photodiode area (not shown). In this example, the color filter layers are formed by depositing the photoresist of the corresponding color and then treating the photoresist with a photo-etching process using a separate mask.
As shown in FIG. 3D, a second planarization layer 111 is formed on the entire surface of the substrate including the color filter layers 107, 108, and 109 and then the second planarization layer 111 is treated with a photo-etching process using a mask so that the second planarization layer 111 remains only on the portion excluding the metal pad region. Referring to FIG. 3E, a micro-lens 112 corresponding to each of the color filter layers 107, 108, and 109 is formed on the second planarization layer 111. Furthermore, the CMOS image sensor fabricated by the above-described method is tested with a probe test in order to check contact resistance. Then, when no problem is detected, the metal pad is electrically connected to the external driving circuit.
However, the above-described known CMOS image sensor and the method for fabricating the same have the following disadvantages. After forming the opening on the metal pad, the first planarization layer, the red (R), green (G), and blue (B) color filter layers, the second planarization layer, and the micro-lens are sequentially formed. Thus, the later processes are carried out while the metal pad is exposed. Accordingly, due to the later processes, the metal pad may be damaged by an alkali solution of a TMAH group, which modifies the metal pad to a hard-type aluminum (Al), thereby increasing the contact resistance, which results in a greater number of failures when carrying out the probe test.
Also, when carrying out the probe test, a deep probing process may be performed, which cuts out the surface of the metal pad in order to reduce the contact resistance. In this case, a large number of the metal pad particles may be generated, which incapacitates the functions of the photodiode, thereby reducing product yield.
In the above-described known image sensor fabrication method, the opening on the metal pad may be formed after forming the micro-lens. However, the color filter layers are formed of photosensitive materials. As a result, when the opening on the metal pad is formed by using a photo-etching process after forming the micro-lens, the color filter layers may be damaged. Therefore, it may not be possible to form the opening on the metal pad after forming the micro-lens.